1. Field of the Invention
Aspects of the present invention relate to a thin film transistor, a method of fabricating the same, and an organic light emitting diode (OLED) display device including the same. More particularly, aspects of the present invention relate to a thin film transistor, in which the concentration of a metal catalyst is adjusted according to the location of a channel region of a semiconductor layer, which is crystallized using a metal catalyst, so that electrical characteristics are excellent, a method of fabricating the same, and an OLED display device including the same.
2. Description of the Related Art
Generally, a polycrystalline silicon layer can be used in a high field effect mobility and high operating circuit, and can constitute a CMOS circuit. Therefore, a polycrystalline silicon layer is widely used for a semiconductor layer of a thin film transistor. The thin film transistor using a polycrystalline silicon layer is generally used for an active element of an Active Matrix Liquid Crystal Display (AMLCD), and a switching element and a driving element of an organic light emitting diode (OLED).
Methods of crystallizing amorphous silicon into polycrystalline silicon include a solid phase crystallization (SPC) method, an excimer laser crystallization (ELC) method, a metal induced crystallization (MIC) method, and a metal induced lateral crystallization (MILC) method. In the SPC method, an amorphous silicon layer is annealed for hours or tens of hours at a temperature of about 700° C. or less, i.e., less than a transition temperature of glass that forms a substrate of a display device in which a thin film transistor is used. In the ELC method, an excimer laser is irradiated onto a silicon layer to locally heat the silicon layer for a very short time period at a high temperature, so that the silicon layer is crystallized. In the MIC method, metals such as nickel, palladium, gold, aluminum, etc., are placed in contact with an amorphous silicon layer or are injected, so that the amorphous silicon layer is changed into a polycrystalline silicon layer, i.e., a phase change is induced. In the MILC method, silicide, which is generated by reacting metal with silicon, is laterally and continuously diffused to sequentially induce crystallization of the amorphous silicon layer.
However, the SPC method takes a long process time, and the annealing process performed at a high temperature for a long time carries a risk of deformation of a substrate. Also, in the ELC method, a high-priced laser device is required, and a protrusion is created on the polycrystalline surface, so that interfacial characteristics between a semiconductor layer and a gate insulating layer may be inferior.
At present, since in a method of crystallizing an amorphous silicon layer using a metal, crystallization can be achieved at a lower temperature and with less time consumption than in the SPC method, research into crystallization methods using metals is actively progressing. Methods of crystallizing an amorphous silicon layer using a metal includes an MIC method, an MILC method, and a Super Grain Silicon (SGS) crystallization method.
One of the factors determining the characteristics of a thin film transistor is leakage current. In particular, in a semiconductor layer that is crystallized using a metal catalyst, the metal catalyst may remain in a channel region to increase the leakage current. Therefore, unless the concentration of the metal catalyst in the channel region is controlled to a predetermined concentration or less, the leakage current of the thin film transistor may be increased and the electrical characteristics of the channel region may be worsened.